The number of E. coli poisoning cases in Sweden rose to 39 on Monday, 15 of which are serious, health officials said.

The victims, who are spread across Sweden, are thought to have been contaminated while visiting Germany, said the Swedish Institute of Infectious Disease Prevention in a statement.

According to a count made Monday afternoon, 15 of the 39 cases have contracted full-blown haemolytic uraemic syndrome (HUS), a disease that causes and serious and which can result in death.

But no deaths from the contamination have been reported thus far in Sweden, said the statement.

With 12 reported deaths in Germany, authorities there believe they may be facing one of the worst E. coli outbreaks in their history. Spanish cucumbers are thought to be the cause, though no provenance has been confirmed.